Alkylaromatic fluids have been proposed as functional fluids where good thermal and oxidative stability are required.
U.S. Pat. No. 4,714,794 (Yoshida) describes monoalkylated naphthalenes as having excellent thermal and oxidative stability, low vapor pressure and flash point, good fluidity, high heat transfer capacity and other properties which render them suitable for use as thermal medium oils.
The use of a mixture of monoalkylated and polyalkylated naphthalenes as a base for synthetic functional fluids is described in U.S. Pat. No. 4,604,491 (Dressler). Pellegrini U.S. Pat. Nos. 4,211,665 and 4,238,343 teaches use of alkylaromatics as transformer oils.
Alkylated naphthalenes are usually produced by alkylating naphthalene or a substituted naphthalene in the presence of an acidic alkylation catalyst such as a Friedel-Crafts catalyst. Acidic clay was used in Yoshida U.S. Pat. No. 4,714,794 while aluminum trichloride was used in Pellegrini U.S. Pat. Nos. 4,211,665 and 4,238,343.
A collapsed silica-alumina zeolite catalyst for alkylation of naphthalene was disclosed in Boucher U.S. Pat. No. 4,570,027. Use of intermediate pore zeolites such as ZSM-5 and large pore zeolites such as zeolite L and ZSM-4 for alkylation of benzene is disclosed in Young U.S. Pat. No. 4,301,316.
For functional fluids based on alkyl naphthalenes, it the preferred alkyl naphthalenes are mono-substituted. They provide the best combination of properties in the finished product and have fewer benzylic hydrogens than corresponding di-substituted or polysubstituted versions and have better oxidative stability. In addition, mono-substituted naphthalenes have a kinematic viscosity in the desirable range of about 5-8 cSt (at 100.degree. C.) when working with alkyl substituents of about 14 to 18 carbon atoms chain length. Although mono-alkylated naphthalenes may be obtained in admixture with more highly alkylated naphthalenes using conventional Friedel-Crafts catalysts or by the use of zeolites such as USY, the selectivity to the desired mono-alkylated naphthalenes is not as high as desired.
Several recent advances have been made in this area which improve the yields of the desired mono-alkylated naphthenes.
U.S. Pat. No. 5,034,563, Ashjian et al, which is incorporated by reference, teaches use of a zeolite containing a bulky cation. The use of, e.g., USY with cations having a radius of at least about 2.5 Angstroms increases selectivity for desired products. Taught as suitable were zeolites containing hydrated cations of metals of Group IA, divalent cations, especially of Group IIA, and cations of the Rare Earths. H, NH4, Na were added to USY zeolite by forming a slurry of zeolite and liquid, 1 hour of stirring, decantation, and a repeat of the exchange procedure.
U.S. Pat. No. 5,177,284, Le et al, which is incorporated by reference, discussed the desirable properties of alkylated naphthalene fluids with higher alpha:beta ratios, including improved thermal and oxidative stability. Le et al found that several parameters influenced the alpha:beta ratio of the alkylated naphthalene products, including steaming the zeolite, lowering the alkylation temperature; or use of acid-treated clay. Steamed USY catalyst gave excellent results in the examples. The patentees mentioned use of zeolites with reduced activity due to base exchange, alkaline earth ion exchange and use of boron-zeolite beta.
U.S. Pat. No. 5,191,135 Dwyer et al, which is incorporated by reference, disclosed co-feeding water when using a large pore zeolite catalyst, such as zeolite Y. Adding from 1-3 wt % water to the feed improved the alkylation reaction, a result attributed to suppression of zeolite acid site activity.
U.S. Pat. No. 5,191,134, Le, which is incorporated by reference, disclosed a similar alkylation process using MCM-41.
We did additional work to see if we could further improve this alkylation process. We wanted to increase the efficiency of the reaction both in terms of conversion and yields.
We discovered that catalyst containing controlled amounts of NH4 and H and rare earths gave unexpectedly superior results. Phrased another way, catalyst with a limited amount of rare earths, exchanged with ammonia and then only about half calcined, gave better results than a like catalyst which was left wholly in the ammonium form, or calcined to be in the protonic form.